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[ CVE-2026-49265 · Published 13 Oct 2026 ]

Oauthlib: Timing Attack Vulnerability in PKCE code_verifier Comparison (CWE-208)

CVSS 6.8 EPSS <1%Fix available

[ Description ]Official description

OAuthLib is an implementation of the OAuth request-signing logic for Python 3.6+. From 3.0.0 until 4.0.0, OAuthLib's PKCE checks in oauthlib/oauth2/rfc6749/grant_types/authorization_code.py compare attacker-supplied code_verifier values with stored challenges using ordinary string equality in code_challenge_method_plain and code_challenge_method_s256. Because the comparison can return after a length difference or the first mismatching character, repeated token endpoint measurements can reveal common-prefix timing information that could potentially allow recovery of the code_verifier, unauthorized access-token acquisition, and account takeover. Practical exploitation requires an intercepted, single-use authorization code and enough measurements to overcome real-world network noise; the local timing benchmark does not establish practical remote account takeover, so the weakness remains a defense-in-depth concern. This issue is fixed in version 4.0.0.

[ Fixes ]Fixes and patches

  • oauthlib 4.0.0Fixed version · GitHub Advisories · pip package oauthlib: upgrade to 4.0.0 or later (affected >= 3.0.0, < 4.0.0)

[ Packages ]Affected open-source packages

[ References ]References